1. Technical Field
The present disclosure relates to an image pickup apparatus for acquiring a spectral image, a spectroscopic system, and a spectroscopic method using the image pickup apparatus and/or the spectroscopic system.
2. Description of the Related Art
Utilization of spectral information for a large number (e.g., several tens or more) of narrow wavelength bands allows grasp of detailed physical properties of an observed object, which cannot be obtained from a conventional RGB image. A camera which acquires information for multiple wavelength bands is called a “hyperspectral camera”. Hyperspectral cameras are used in all fields, such as food inspection, living body inspection, drug development, and mineral component analysis.
As an example of utilization of information for a limited narrow wavelength band, International Publication No. 13/002350 discloses a device which discriminates between a tumor site and a non-tumor site in a subject. The device detects, through irradiation with excitation light, fluorescence at 635 nm emitted from protoporphyrin IX which is accumulated in cancer cells and fluorescence at 675 nm emitted from photo-protoporphyrin, thereby distinguishing between a tumor site and a non-tumor site.
Japanese Unexamined Patent Application Publication No. 2007-108124 discloses a method for judging the freshness of perishable food that lowers over time by acquiring information on reflectance characteristics of light with continuous multiple wavelengths.
Hyperspectral cameras capable of acquiring images and/or measuring reflectance for multiple wavelengths are broadly divided into the four types:    (a) a line sensor type;    (b) an electronic filter type;    (c) a Fourier transform type; and    (d) an interference filter type.
In a hyperspectral camera of the line sensor type in (a), one-dimensional information on an object is acquired using a member having a linear slit. Light after passage through the slit is separated by a dispersive element, such as a diffraction grating or a prism, in accordance with wavelength. Separated light components of different wavelengths are detected by an image pickup device (e.g., image sensor) having a plurality of two-dimensionally arrayed pixels. Under this system, only one-dimensional information on an object to be measured is obtained at one time. Two-dimensional spectral information is scanned by operating the whole camera or the object to be measured perpendicularly to a direction of the slit. The line sensor type has the advantage that high-resolution images for multiple wavelengths are obtained. Japanese Unexamined Patent Application Publication No. 2011-89895 discloses an example of the hyperspectral camera of the line sensor type.
Hyperspectral cameras of the electronic filter type in (b) fall into a type using a liquid crystal tunable filter (LCTF) and a type using an acousto-optic tunable filter (AOTF). A liquid crystal tunable filter is an element with multiple tiers, each having a linear polarizer, a birefringent filter, and a liquid crystal cell. The liquid crystal tunable filter can eliminate light of an unnecessary wavelength and extract only light of any specific wavelength only through voltage control. An acousto-optic device is composed of an acousto-optic crystal and a piezoelectric element, which are bonded with each other. When an electrical signal is applied to the acousto-optic crystal, ultrasonic waves are generated, and thereby compressional standing waves are formed within the crystal. With a diffraction effect of the standing waves, the acousto-optic device can extract only light of any specific wavelength. Although a target wavelength is limited, the electronic filter type has the advantage of being able to acquire high-resolution moving image data.
A hyperspectral camera of the Fourier transform type in (c) uses the principle in a two-beam interferometer. A light beam from an object to be measured is split into light beams by a beam splitter, and the light beams are reflected by a fixed mirror and a movable mirror, respectively, and are coupled again. A coupled light beam is then observed by a detector. Data indicating a change in the intensity of interference dependent on a light wavelength can be acquired by varying the position of the movable mirror over time. A Fourier transform is performed on the obtained data to obtain spectral information. The Fourier transform type has the advantage of being able to simultaneously acquire pieces of information for multiple wavelengths.
A hyperspectral camera of the interference filter type in (d) is of a type using the principle in a Fabry-Perot interferometer. The interference filter type uses a configuration in which optical elements having two faces with high reflectance which are separate by a predetermined distance are arranged on a sensor. Distances between the two faces of the optical elements differ from region to region and are determined so as to meet an interference condition for light of a desired wavelength. The interference filter type has the advantage of being able to simultaneously acquire pieces of information for multiple wavelengths as a moving image.
Besides the types, there is available a type using compressive sensing, as disclosed in, for example, U.S. Pat. No. 7,283,231. A device disclosed in U.S. Pat. No. 7,283,231 disperses light from an object to be measured with a first dispersive element such as a prism, marks the dispersed light with a coded mask, and restores a light ray path with a second dispersive element. With this configuration, an image which is obtained through coding and multiplexing on a wavelength axis is acquired by a sensor. A plurality of images for multiple wavelengths can be reconstructed from the multiplexed image through use of compression sensing.
Compressing sensing refers to a technique for restoring, from a small number of pieces of data acquired as samples, pieces of data larger in number. Assuming that (x,y) is two-dimensional coordinates of an object to be measured and λ is a wavelength, data f desired to be obtained is the three-dimensional data (x,y,λ). In contrast, image data g obtained by a sensor is two-dimensional data which is compressed and multiplexed in a λ axis direction. The problem of obtaining the data f larger in data volume from the acquired image g smaller in data volume is a so-called ill-posed problem and is impossible to solve in this state. However, data of a natural image generally has redundancy, and skillful utilization of the redundancy allows conversion of the ill-posed problem into a well-posed problem. An example of a technique for reducing data volume using the redundancy of an image is JPEG compression. JPEG compression uses the process of converting image information into a frequency component and removing a non-essential portion of data (for example, a component with low visibility). In compressive sensing, the above-described technique is incorporated into arithmetic processing, and a desired data space is converted into a space represented with redundancy. With this conversion, unknowns are reduced, and a solution is obtained. For the conversion, for example, a discrete cosine transform (DCT), a wavelet transform, a Fourier transform, or total variation (TV) is used.